The present invention relates to an information recording medium capable of recording, storing and reproducing electrostatic information in the form of visible information by using an ultraviolet curing resin material having a liquid crystal material dispersed and fixed therein. The present invention also relates to an information recording and reproducing method that employs the above-described information recording medium.
Liquid crystal display devices have been used as display devices for office automation equipment such as word processors, laptop computers, etc. These liquid crystal display devices have a liquid crystal layer sandwiched between two transparent substrates, e.g., glass or plastic substrates, provided with respective transparent electrode films. A voltage is applied between the two electrodes by using either simple- or active-matrix drive, thereby displaying information, e.g., an image.
This type of liquid crystal display device includes typical TN and STN liquid crystal display devices and also those based on dynamic scattering mode, i.e., liquid crystal display devices which operate on the current effect, and those which utilize the cholesteric-nematic phase transition. Recently, liquid crystal display devices having a liquid crystal material combined with a polymer have also been used. A liquid crystal display device having a nematic liquid crystal dispersed and fixed in a polymeric resin material displays information according to whether it is transparent or opaque by making use of the phenomenon explained below: The ordinary ray refractive index of the liquid crystal material and the refractive index of the polymeric resin material have previously been made equal to each other. When a voltage is applied to the device to align the molecules of the liquid crystal material in the direction of the voltage application, the device becomes transparent. When no voltage is applied to the device, the molecules of the liquid crystal material are irregular in orientation. Consequently, light is scattered at the interface between the liquid crystal and the polymeric resin material or in the liquid crystal dispersed and fixed therein. Thus, the liquid crystal display device becomes opaque.
A liquid crystal display device having a nematic liquid crystal dispersed and fixed in a polymeric resin material has the following advantageous features: the display area can be increased; the response time, particular, the rise time, is short; no polarizer is needed, and hence the light utilization efficiency is high; the viewing angle is wide and uniform; it has flexibility; and so forth. It has recently been reported that this type of liquid crystal display device is used as a lighting control sheet for a window, a projection-type display, etc. This type of liquid crystal display device has an advantage in that the decay time is as short as 1 ms to 30 ms, which is shorter than that of TN and STN display devices, which are most commonly used as projection-type displays and computer displays.
In a conventional liquid crystal display device having a nematic liquid crystal dispersed and fixed in a polymeric resin material, an AC voltage is applied between two electrodes sandwiching the nematic liquid crystal, and information is displayed and erased by on/off control of the voltage application. In this case, when the application of the AC voltage is cut off, i.e., when no voltage is applied, the device returns to the initial transmittance, that is, the opaque state, although there is a report stating that slight hysteresis is observed in the change of light transmittance relative to the applied voltage; therefore, there is a difference between the transmittance when the applied AC voltage is rising and the transmittance when the voltage is falling.
A common method of displaying information, e.g., an image, by a liquid crystal display device is to on/off control a liquid crystal material sandwiched between two matrix electrodes for each pixel. Other conventional displaying methods include one that employs an imagewise patterned electrode, and another wherein when a composite film of a polymer and a liquid crystal is to be formed, a polymeric material of the ultraviolet curing type, for example, is used and this is imagewise irradiated with ultraviolet rays. In these conventional methods, however, fixed information is displayed by on/off control.
Meantime, there are liquid crystal display devices having memory effect. One type of such liquid crystal display device uses cholesteric-nematic phase transition. This type of liquid crystal display device must have a sandwich structure in which a liquid crystal material is sandwiched between transparent electrodes formed with respective vertical alignment films. In this case, a proper electrode structure is required.
Recently, an information recording medium which uses a polymer dispersed liquid crystal in place of a liquid crystal layer in a liquid crystal cell has been developed. In such an information recording medium, a polymer dispersed liquid crystal is sandwiched between two substrates having respective ITO electrodes.
There has also been developed an information recording technique in which an information recording medium, which is composed of a substrate, an ITO electrode and a polymer dispersed liquid crystal layer, is disposed to face a photoelectric sensor composed of an electrode and a photoconductive layer, thereby forming an information recording system, and information light for exposure is applied to the information recording system with a voltage applied between the two electrodes, thereby allowing a voltage corresponding to the information light to be applied to the polymer dispersed liquid crystal layer, and thus recording the desired information in the form of liquid crystal alignment in the polymer dispersed liquid crystal layer. In this case, when the liquid crystal dispersed in the resin material is a smectic liquid crystal, particularly large memory effect and orientational order parameter are obtained. Accordingly, even if the electric field is removed, the modulated alignment is maintained as it is. Thus, the recorded image can be read even after the information recording medium has been allowed to stand for a long time. Since the visible image recorded on the medium is erased by heating it to a temperature in the vicinity of the isotropic phase transition temperature of the liquid crystal, the information recording medium can be reused.
In this type of information recording medium, however, the liquid crystal oozes from the surface of the information recording layer, causing noise to be generated. Therefore, it is necessary to suppress oozing of the liquid crystal from the information recording layer surface. In addition, if the liquid crystal oozes out, an electrode layer which is provided on the surface of the information recording layer is likely to peel off. Thus, the conventional information recording medium is demanded to improve in durability during storage. Furthermore, if an electrode layer is provided on the information recording layer by vapor deposition, the surface of the information recording layer is also demanded to have durability, i.e., sputtering resistance.
In addition, the conventional information recording medium of the type in which information is recorded in the form of liquid crystal alignment in the polymer dispersed liquid crystal layer generally suffers from the problem that a satisfactorily high contrast cannot be obtained between recorded and unrecorded areas of the information recording layer, resulting in lack of signal.
It is a first object of the present invention to provide an information recording medium which is free from oozing of the liquid crystal from the surface of the information recording layer, superior in durability and capable of obtaining an image of high quality without occurrence of noise in information recording, and also provide an information recording and reproducing method that employs the above-described information recording medium.
It is a second object of the present invention to provide an information recording medium of the type described above, which enables information of high contrast ratio to be obtained.